You're looking for the Peterson Field Guide to Edible Wild Plants.
http://books.google.com/books/about/A_Field_Guide_to_Edible_Wild_Plants_of_E.html?id=ce1G-Nr9pVoCAlso check out the list of related books.
I'm still looking for a version of the guide in electronic format. I always have my Android with me when I'm out camping, but bringing extra books is just added weight.